Defender is a computer adaptation of the arcade game with the same name. Like the original it's a scrolling shoot 'em up game where it's possible to move both left and right. The objective of the game is to protect earthlings that aliens are trying to capture and turning into mutants. A radar shows the locations of the aliens and the player's job is to shoot them down before they get away with the humans. A captured earthling can still be rescued by shooting the alien and then catching the earthling and returning it to the ground. In addition to the regular fire there are also smart bombs that destroys everything on the screen and a hyperspace option for getting out of trouble.